1. Field of the Invention
The present invention relates to an automatic focusing device for use in a camera.
2. Description of the Prior Art
An automatic focusing mechanism, or auto-focusing mechanism it is as often referred to, is built in many of modern video cameras or similar cameras of the type using a solid-state image sensor, such as a CCD (charge coupled device) array. A variety of approaches have heretofore been proposed to implement automatic focusing such as emitting an infrared ray, ultrasonic wave or similar radiation from a camera to a desired object and then determining the distance between the camera and the object on the basis of a reflection from the object which is incident to a built-in sensor of the camera; measuring the distance from a camera to an object by use of a range finder which adopts the principle of a trigonometrical survey; and using a particular component of a video signal which is representative of the contrast of a picture, i.e., contrast signal so as to determine a position where the contrast signal has the largest value to be the focus.
The auto-focusing mechanism using a contrast signal as stated above may be constructed to separate only a high-frequency component from an image signal, convert the high-frequency component into a DC level by peak-to-peak detection, and focus a lens system in such a manner as to maintain the signal produced by the conversion at the largest value at all times, as disclosed in Japanese Patent Laid-Open Publication No. 12615/1973 by way of example. The device disclosed in this Laid-Open Publication is mainly constituted by a local extraction circuit for extracting a video signal which appears in a particular focusing area which is defined on a picture plane, a circuit for producing a signal representative of a contrast from the extracted video signal by an arithmetic operation, and a servo mechanism for maintaining the largest value of the the contrast signal by adjusting the focus of a lens. The local extraction of a video signal is accomplished by marking a certain area (usually a central area) of a single picture plane in the form of a rectangle or a circle, for example, the focus being adjusted by a video signal extracted from such an area.
The prior art method for the extraction of a video signal stated above is satisfactory so long as an object to be picked up has a pattern which bears a contrast distribution in the horizontal direction of a video signal, e.g. a vertical stripe pattern. However, it is not satisfactory when it comes to an object having a pattern which has a contrast distribution in the vertical direction, e.g. a horizontal stripe pattern. Further, since the focusing operation is performed in a video frequency band which is as high as 3 megahertz or 2 megahertz, for example, a processing circuit has to be implemented by high-speed operating elements which are undesirable from the standpoint of reliable operation and light-weight and miniature configuration of an auto-focusing device.